1. Field of the Invention
The present invention relates to a rotatory polarization plate. This invention further relates an optical element using the rotatory polarization plate and a polarizing element. This invention further relates to a light condensation backlight system using the optical element and still further to a liquid crystal display using the same.
2. Description of the Related Art
There has been conventionally conducted a trial to condense or collimate light from a diffusion light source using an optical film having a flat front surface or to control a transmittance of light therefrom in a specific direction of the optical film having a flat front surface. A typical example of such a trial is a method in which a bright line light source is combined with a band pass filter (see, for example, a publication of JP-A No. 6-235900, a publication of JP-A No. 2-158289, a publication of JP-A No. 10-321025, a specification of U.S. Pat. No. 6,307,604, a specification of DE 3836955 A, a specification of DE 4222028 A, a specification of EP 578302 A, a specification of USP 2002/34009 A and a pamphlet of WO 02/25687 A1). There has been proposed a method in which a band pass filter is disposed on a CRT, or a display with a light source emitting a bright line such as electroluminescence to thereby condense and collimate light; or the like (see, for example, a pamphlet of WO 98/49585 A1, a pamphlet of WO 99/09452 A1, a specification of USP 2002/036735 A, a publication of JP-A No. 2002-90535 and a publication of JP-A No. 2002-258048).
A method has been proposed in which polarization and retardation are combined with each other or the like (see a specification of JP No. 2561483). An optical element has been proposed, in other patent literatures, that is constituted of a reflection polarizer-a rotatory polarization plate-a reflection polarizer (see a specification of U.S. Pat. No. 4,984,872, a specification of USP 2003/63236 A and a pamphlet of WO 03/27731 A1). An optical element has been proposed that uses a hologram material (see a pamphlet of WO 03/27756 A1).
In a method in which a bright line spectrum is used as an optical film imparting directivity to a diffusion light source, however, since a requirement is a high precision level related to wavelength matching between a kind of a light source and a band pass filter, which has made fabrication thereof difficult. On the other hand, no large problem occurs in a case where a monochromatic light is used, whereas in a case where adaptation is required for the three primary colors, coloration is felt unless transmittance of the colors changes at the same ratio according an incidence angle. Therefore, in combination of a bright line light source and a band pass filter, a requirement is a precise matching of a wavelength of the light source with a band pass filter, which is high in technical difficulty.
For example, in the publications of JP-A 2002-90535 and JP-A 2002-258048, used for light condensation in the front direction is a reflecting plate obtained by combining a left circularly polarized light separating plate and a right circularly polarized light separating plate together or alternatively a reflecting plate obtained by inserting a ½ wavelength plate between circularly polarized light separating plates with the same direction of the rotation. In this system, a necessity arises for forming corresponding layers for respective wavelengths of a light source, which necessitates lamination of three sets for color display. This has led to complexity in construction and a high cost.
In a case where polarization and retardation are used, there has arisen a tendency that a secondary transmission region emerges at a further larger incidence angle if an emittable angle range is narrowed.
In a case where obliquely incident light passes through a retardation plate, there is generally a tendency that an optical path length is longer; and with an increase in optical path length, a difference in optical path length also increases. With combination of this characteristic and a polarizer adopted, it is possible to fabricate a polarizing element having angular dependency of transmittance as taught in the specification of JP 2561483. Such a polarizing element can change a transmittance according to an incidence angle. For example, with such a polarizing element, it is possible that a transmittance in the front direction is higher, while a transmittance of an obliquely incident light is lower.
If a layer imparting no retardation in the front direction and a retardation of ½ wavelength in an oblique direction is inserted between optical elements separating circularly polarized light in the same direction of the rotation, light is totally reflected in an oblique direction; therefore, light is transmitted only in the front direction (see Publication of JP-A No. 10-321025). In this method, however, in a case where a condition that total reflection occurs at a specific angle is set, a problem has been remained that a transmission region emerges at an incidence angle larger than the specific angle. With increase in incidence angle, the length of an optical path is longer and an imparted retardation increases. Hence, a property emerges that light is again transmitted at an incidence angle that imparts a retardation of a ¾ wavelength. Therefore, if a transmission characteristic is confined only in the front direction, a transmission component is, to the contrary, generated in an oblique direction, which has become a trouble.
The specification of USP 2003/63236 and the pamphlets of WO 03/27731 A1 and WO 03/27756 A1 all improve a productivity of reflection polarizer laminates for use in transflective by enabling a production according to a roll-to-roll method by use of a rotatory polarizer, as solution of a problem of reduction in productivity and decrease in area yield which have been caused by fabricating the reflection polarizer laminates through lamination having a small displacement in angular registration. In such a general combination of a reflection polarizer-a rotatory polarization plate-a reflection polarizer, there has been no chance that an angular dependency of a transmittance occurs. In a general polarizer using a laminate of a chiral material and a retardation plate such as quartz crystal and saccharose, it is difficult to fabricate the rotatory polarizer, while intentionally controlling a retardation plate having a rotatory polarization characteristic changed by an incidence angle.
It has been generally known that a rotatory polarization effect can be obtained by disposing a ½ wavelength plate and controlling an angle of the axis direction relative to the polarization axis of incident polarized light. A ½ wavelength plate, however, exerts a rotatory polarization function to vertically incident polarized light, whereas the plate cannot control a rotatory polarization angle uniformly for polarized light passing through in oblique directions. This is because a ½ wavelength plate itself has the axis thereon; therefore, an azimuth angle of the polarization axis relative to the axis of the plate is different according to an incidence angle. A rotatory polarizer using a chiral material has been known. Since, in the rotatory polarizer, a rotatory polarization angle is changed simply by an optical path length, a rotatory polarization angle keeps on increase with increase in incidence angle, and phenomenon cannot be observed that a rotatory polarization angle is settled at a particular value, which is desired. In this way, a retardation plate and a rotatory polarizer that have been conventionally available have had no opportunity to enable a transmittance of polarized light to be controlled by an incidence angle.
On the other hand, hologram materials are, in more of cases, expensive, poor in mechanical characteristics, and soft and weak in nature, which have been problematic about long term durability.
As discussed above, conventional optical elements have been problematic because of difficulty in fabrication, hardness in obtaining a target optical characteristic, poor reliability and the like.